The ultrastructure of the serotonin (5HT) system in the spinal cord of rats was studied by an immunohistochemical peroxidase-antiperoxidase (PAP) method. Under the light microscope, 5HT immunoreactive staining was observed as brown-colored dots in the anterior horn, lateral horn, posterior horn and pericentral canal region. These positively staining dots were probably indicative of 5HT immunoreactive varicosities and nerve terminals. At the ultrastructural level, 5HT immunoreactive nerve fibers appeared as darkly stained varicosities with PAP positive large electron dense vesicles (80-100 nm), as well as small clear vesicles (30-40 nm) finely coated with PAP immunoreactive products. In the anterior horn, some of the 5HT immunoreactive structures were clearly nerve terminals forming asymmetric synaptic contact with soma or dendrites of the anterior horn cells. In the lateral horn, posterior horn and pericentral canal region, however, only 5HT positive varicosities were detected.</p
